1. Field of Invention
The present invention relates to frequency-controlled inverters having series-tuned L-C output circuitry, particularly of a kind wherein control of frequency provides control of output power in such manner as to compensate for effects of variations in the magnitude of the DC supply voltage and/or in the loading provided by way of the L-C output circuitry.
2. Elements of Prior Art
In inverter-type fluorescent lamp ballasts using a series-excited parallel-loaded resonant L-C circuit for matching the inverter's output to the fluorescent lamp, an important problem relates to possible damage of the inverter in case the series-excited parallel-loaded resonant L-C circuit is left unloaded, such as may occur in the fluorescent lamp were to be removed. To prevent such damage from occurring, it is necessary to provide means whereby the maximum power drawn from the inverter be manifestly limited to a safe level. For instance, such limitation is accomplished by Zansky in U.S. Pat. No. 4,392,087 by using the inverter's DC supply as an alternative load which activates as soon as the output voltage exceeds a certain magnitude. Or, in U.S. Pat. No. 4,398,126 to Zuchtriegel, inverter protection is provided by way of disabling the inverter in case the L-C circuit is left unloaded.